The present disclosure relates generally to integrated circuit (IC) manufacturing, and more particularly to an improved method and system for examining mask fidelity to determine an appropriate method to create a mask.
Photolithography is one of the principal processes in the manufacture of semiconductor devices, and consists of patterning the wafer's surface in accordance with the circuit design of the semiconductor devices to be produced. More specifically, a circuit design to be fabricated on the wafer is first patterned on a mask or reticle. The wafer is coated with a photo resist material, and is then placed in a photolithography tool to be exposed to light passing through the reticle to produce a latent image of the reticle on the photo resist material. Thereafter, the exposed photo resist material is developed to produce the image of the mask on the wafer. After the completion of the photolithography process, the uppermost layer of the wafer is etched, a new layer is deposited, and the photolithography and etching operations are started again. In this repetitive manner, a multi-layer semiconductor wafer is produced.
As is well known, photolithography tools utilize a lamp or a laser as a light source, and utilize a relatively high numerical aperture (NA) objective to achieve a relatively high resolution. The optics of such tools are generally designed to produce reduction (negative magnification) of the image of the reticle onto the wafer. In order to obtain operating semiconductor devices, the reticle must be defect free. Moreover, in most modern processes, the reticle is used in a repeated manner to create many dies on the wafer. Therefore, various reticle inspection tools have been developed and are available commercially.
During the photolithography process, certain entities on the mask will be distorted or lost altogether. This is referred to generally as a fidelity issue. It includes phenomenon such as line end shortening, corner rounding, and small serif disappearance, etc. Some of those are caused by errors on the masks themselves, while others can be caused by processing mistakes. When generating an actual photo mask from a digital mask design, a mask fidelity problem may occur. When circuits on the wafers are made from such a mask, certain errors will then show on the wafer.
It should be appreciated by those skilled in the art that to produce an operational microelectronic circuit, a mask must be as defect-free as possible, preferably completely defect-free. Therefore, mask inspection tools are needed to detect various defects in the masks that can potentially reduce the microelectronic circuit fabrication yields. Smaller feature sizes on the masks used in the photolithographic process, as well as the use of OPC masks, require more sensitive tools for mask inspection. Numerous systems for mask inspection have been developed in response to the growing demands for inspecting mask fidelity problems.
The earliest automated inspection tools for detecting mask errors utilized a technique termed die-to-die inspection where the acquired images of a die on the mask are compared to corresponding images of a second die from the same mask. Any difference between one die to the other die indicated the presence of a defect. The technique was limited in that certain mask surface defects (called “surface” defects, for example, a particle on the surface of a mask) could remain undetected and later appear in a critical mask region after handling of the mask.
Moreover, defects can be detected by inspecting the mask using the image of the mask produced by the light transmitted through the mask and the light reflected by one face of the mask. The mask inspection tool that uses this method acquires both images then analyzes the images. The results of the analysis of the two images yield information on the condition of the mask. The image analysis method may use die-to-die comparison, die-to-database comparison, or reflected image to transmitted image comparison. In the die-to-database method, the acquired die images from the mask images are compared to images that are simulated using the mask design specifications.
Such an inspection system can detect defects that may or may not print on the photo resist during the actual photolithographic process. The major drawback of this method is that it studies the physical structure of the mask independently of the optical image actually produced by the mask on the wafer. For instance, variations in the line width of the image that the mask produces frequently are higher than the corresponding variation in the line width of the mask itself. It is desirable, therefore, to relate the physical structure of the mask to the actual image that the mask creates on the photo resist, and to study directly the image that the mask actually produces.
In order to facilitate the evaluation of the mask performance at the wafer level, tools have been developed that are able to scan a mask and yield an aerial image of the mask as it would appear at the wafer plane. According to this method, the mask inspection system replicates an optical exposure tool's critical parameters used during the exposure of the photo resist during semiconductor device fabrication. The mask inspection device then applies a set, or a plurality of sets, of exposure conditions that may be used in the actual photolithographic process to create an aerial image, or plurality of images, from the mask. In particular, these systems match the wavelength, the partial coherence of the exposure light, illumination aperture and the imaging numerical aperture (NA) of the optical exposure system. The created aerial image is typically magnified and detected using a CCD camera that is sensitive to the ultraviolet radiation. The use of the aerial imaging method permits the detection of the mask defects that would print during the actual photolithographic process. The acquired aerial images are analyzed using software algorithms developed for defect identification.
The inspection methods based on die-to-database comparison that are used by the existing aerial imaging systems are not always effective, especially for highly complicated mask designs. The die-to-database comparison method uses models describing the behavior of an optical exposure system used in the mask manufacturing process to produce the simulated image used in the mask inspection. However, various optical and mechanical factors during the mask making process will impact the final mask. As a result, there are limitations in the accuracy of the transformation from database to simulated aerial image. In addition, after the sequence of writing, developing and etching the photo mask, certain errors may be present on the photo mask or in the realized photo mask layout which are not readily detectable as mask defects. For example, variations in the line widths of the image that the photo mask produces at the wafer plane are frequently higher than the corresponding variations in the line widths of the mask itself. This phenomenon is referred to as the Mask Error Enhancement Factor (MEEF). In effect, the MEEF describes the amplification of reticle errors realized on the wafer surface. This MEEF effect is most noticeable when the lithography involves resolving features on a semiconductor wafer which are smaller than the exposing wavelength of the light used by the exposure tool which forms the patterns on the wafers. The mask defect inspection method utilized by the mask vendor, typically specified by the customer, is often the last automated layout inspection a mask receives prior to use in the wafer facility. After receipt at the wafer fabrication facility, the mask is used to image semiconductor wafers for production of semiconductor devices
While photo mask manufacturers strive to deliver zero-defect photo masks to their customers, there is a certain limit in their photo mask inspection capabilities. From the perspective of photo mask manufacturers, the ideal goal would be to create a circuit on the wafer that closely mimic what is in a digital design in a database. What is needed is an improved method and system for detecting mask fidelity problem so that it can be determined how an appropriate mask should be created.